There are some situations (like when the assembly is executed like an office plugin) that Assembly.GetEntryAssembly = null. When this happens, the bootstrapper adds a null reference in the AssemblySource singleton and the rest of the framework doesn't check for null when consuming this list =(

I fixed that in this commit:
https://github.com/victorarias/Caliburn.Micro/commit/819d19c28fb71b55b4be22fb81d190f9dd268478

It's a very simple fix to a major(?) problem.

PS: I've tried to fork here in codeplex but apparently I don't have permission to push to my own fork...